Test polynomial GCP and TPS warpers in multi-threaded context

git-svn-id: https://svn.osgeo.org/gdal/trunk@27848 f0d54148-0727-0410-94bb-9a71ac55c965
autotest: remove PAM file if existing

git-svn-id: https://svn.osgeo.org/gdal/trunk@27849 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_vc12